Conventionally, to improve the frequency use efficiency, a communication scheme is studied that transmits different signals from a plurality of antennas and obtains signals transmitted by interference compensating means upon reception. This communication scheme is generally referred to as a MIMO (Multi-Input Multi-Output) communication scheme.
FIG. 1 shows a basic schematic configuration diagram for illustrating basic operation of a conventional MIMO communication scheme that transmits different code-divided signals from a plurality of antennas. FIG. 1(a) illustrates a principle of MIMO communication. FIG. 1(b) shows an equation expressing the relationship between transmission signals and received signals. Furthermore, both the transmitting apparatus and receiving apparatus of the MIMO communication scheme shown in FIG. 1(a) have two antennas.
In FIG. 1, the signals transmitted from the antennas are referred to as TX1 and TX2. Further, the signals received by the antennas are referred to as RX1 and RX2. As shown in FIG. 1(b), RX1 and RX2 can be expressed by the following equations 1 and 2.RX1=ATX1+BTX2  (Equation 1)RX2=CTX1+DTX2  (Equation 2)
Here, A is a channel characteristics between transmitting antenna 1 and receiving antenna 1, B is a channel characteristics between transmitting antenna 2 and receiving antenna 1, C is a channel characteristics between transmitting antenna 1 and receiving antenna 2, and D is a channel characteristics between transmitting antenna 2 and receiving antenna 2.
Here, to receive transmission signals TX1 and TX2 from a received signal by estimating four channel characteristics A, B, C and D, using the estimated channel characteristics of four channel characteristics A, B, C and D and performing processing of the following equation 3, it is possible to receive signals TX1 and TX2 transmitted from the antennas.
                                                                                                              DRX                    ⁢                                                                                  ⁢                                          1                      /                                              (                                                  AD                          -                          BC                                                )                                                                              -                                      BRX                    ⁢                                                                                  ⁢                                          2                      /                                              (                                                  AD                          /                          BC                                                )                                                                                            =                                                      D                    ⁡                                          (                                              ATX                        ⁢                                                                                                  ⁢                        1                                            )                                                        +                                      BTX                    ⁢                                                                                  ⁢                    2                                                              )                        /                          (                              AD                -                BC                            )                                -                                    B              ⁡                              (                                                      CTX                    ⁢                                                                                  ⁢                    1                                    +                                      DTX                    ⁢                                                                                  ⁢                    2                                                  )                                      /                          (                              AD                -                BC                            )                                      =                                            (                                                ADTX                  ⁢                                                                          ⁢                  1                                +                                  BDTX                  ⁢                                                                          ⁢                  2                                -                                  BCTX                  ⁢                                                                          ⁢                  1                                -                                  BDTX                  ⁢                                                                          ⁢                  2                                            )                        /                          (                              AD                -                BC                            )                                =                      TX            ⁢                                                  ⁢            1                                              (                  Equation          ⁢                                          ⁢          3                )            
Non-Patent Document 1, for example, studies performing retransmission in such a MIMO communication scheme. Non-Patent Document 1 studies the following two retransmitting methods.
First, retransmitting method 1 collectively encodes data to be transmitted from the antennas and retransmits all data transmitted from all antennas.
FIG. 2 illustrates retransmitting method 1 in a MIMO communication scheme, and shows a frame configuration of data including retransmission data to be transmitted by a transmitting apparatus. As shown in FIG. 2, with retransmitting method 1, the retransmission information requires better quality than normal transmission data, and therefore a null signal is transmitted from another antenna when retransmission data is transmitted, so as to improve quality of the retransmission information.
Thus, when retransmitting the transmission signals, a transmitting apparatus of retransmitting method 1 transmits from all antennas all transmission signals which are collectively encoded and transmitted from all antennas.
Further, retransmitting method 2 performs encoding per data to be transmitted from the antennas and performs retransmission per antenna. This retransmitting method 2 can be implemented by using a transmitting apparatus having the same configuration as the transmitting apparatus of retransmitting method 1 other than the configuration of assigning transmission data per antenna and independently performing encoding processing on the transmission antenna per antenna. Non-Patent Document 1: “A Study of MIMO-OFDM with Error Detection Code System” IEICE Technical Report CAS2003-124, March 2004